Field of the Invention
The present invention relates generally to AC to DC power converters or rectifiers. More specifically, the present invention relates to techniques for reducing or eliminating the need for aluminum electrolytic capacitors in power circuits using metalized film capacitors.
Description of the Related Art
The reliability of today's Alternating Current (AC) to Direct Current (DC) converter, or rectifier, is based on many factors including individual component reliability, stress de-rating, operational temperature, and manufacturing processes.
One of the major known issues with component reliability is the use of aluminum electrolytic capacitors found in current designs. This type of capacitor is used in applications where a high capacitance to volume ratio is beneficial. In single phase AC to DC power converter systems, line voltage varies sinusoidally from a maximum value to zero at the line frequency. At zero voltage power cannot be generated, while at low voltages little power is available. In modern power converter designs there is usually a requirement to have a power factor close to unity operating at a high efficiency, where power factor is defined as the ratio of real power to apparent power (real plus reactive power). Power delivered varies at the line frequency on the AC side of the converter.
On the DC side, power is normally constant in most applications, such as a Photovoltaic (PV) solar panel array or a battery charger. In order to transform constant power into variable power, some form of energy storage mechanism is involved. In today's conventional single phase converters, a bank of aluminum electrolytic capacitors situated is provided for storing energy, or acting as a power sink, when excess power is available. This arrangement supplies energy, or acts as a power source, when needed. In general, an aluminum electrolytic capacitor tends to be an economical choice. However, this type of capacitor has a known wear characteristic due to the problematic drying out of the inherent liquid electrolyte. This wear characteristic limits the lifetime of electrolytic based capacitors to around 10 years in a conventional converter.
Recently, there has been a surge in the use of transformer-free power inverters. This type of inverter does not include an isolation transformer. No intermediate DC link capacitor is provided, but a DC capacitor is positioned in parallel with the DC input originating from the solar panel array, or battery. In this arrangement, the value of capacitance increases since the panel voltage is known, in the solar panel array context, to vary as a function of the solar irradiance incident on the solar panel array surface. Current operating voltages in such solar panels can vary from 100 to 400 volts DC in a typical PV string. In order for these designs to deliver sufficient power without a large current ripple at low voltages, a large number, or bank formation, of capacitors must be provided.
Since electrolytic capacitors have known reliability issues, many designers eliminate them from consideration. For example, the electric vehicle industry has already discontinued use of electrolytic capacitors in product implementations. This was made possible because the inverters required for EV traction drives are provided with DC from a battery and convert DC to 3-phase AC for electric motor propulsion, resulting in low energy storage requirements. In the photovoltaic industry, the application is DC provided from the solar array conversion to single phase AC output, for example supplying power to a home or returning power to the AC grid, where energy storage requirements for such photovoltaic/solar array applications are much larger.
It would therefore be advantageous to provide a system that overcomes known component lifetime limitations in power circuit designs while improving overall system reliability.